Loss of β1-Integrin Enhances TGF-β1-induced Collagen Expression in Epithelial Cells via Increased αvβ3-Integrin and Rac1 Activity

Transforming growth factor beta(TGF-beta) promotes tissue fibrosis via the receptor-specific Smad pathway and non-canonical pathways. We recently reported that TGF-beta 1-stimulated collagen expression by cultured kidney cells requires integrin-dependent activation of focal adhesion kinase (FAK) and consequent ERK MAP kinase activity leading to Smad3 linker region phosphorylation. Here, we defined a role for alpha v beta 3-integrin in this non-canonical pathway. A human kidney tubular cell line in which beta 1-integrin was knocked down (beta 1-k/d) demonstrated enhanced type I collagen mRNA expression and promoter activity. A second shRNA to either alpha v-integrin or beta 3-integrin, but not to another alpha v-binding partner, beta 6-integrin, abrogated the enhanced COL1A2 promoter activity in beta 1-k/d cells. Although alpha v beta 3-integrin surface expression levels were not different, alpha v beta 3-integrins colocalized with sites of focal adhesion significantly more in beta 1-k/d cells, and activated alpha v beta 3-integrin was detected only in beta 1-k/d cells. Further, the collagen response was decreased by a function-blocking antibody or a peptide inhibitor of alpha v beta 3-integrin. In cells lacking alpha v beta 3-integrin, the responses were attenuated, whereas the response was enhanced in alpha v beta 3-overexpressing cells. Rac1 and ERK, previously defined mediators for this non-canonical pathway, showed increased activities in beta 1-k/d cells. Finally, inhibition of alpha v beta 3-integrin decreased Rac1 activity and COL1A2 promoter activity in beta 1-k/d cells. Together, our results indicate that decreasing beta 1 chain causes alpha v beta 3-integrin to become functionally dominant and promotes renal cell fibrogenesis via Rac1-mediated ERK activity.